Method and device for electronically controlling a vehicle deceleration in dependence on a differential slip between two vehicle axles

ABSTRACT

A method for controlling a vehicle deceleration depending on a differential slip between two vehicle axles in a vehicle with an ABS brake system includes detecting at least one of a target vehicle deceleration specified by the driver and an actual vehicle deceleration; and controlling a braking pressure on wheel brakes of a vehicle axle to be controlled by actuation of ABS brake valves in such a way that the braking pressure on the wheel brakes of the vehicle axle to be controlled is controlled depending on a detected actual differential slip, so that the actual differential slip corresponds to a target differential slip. The actual differential slip indicates a difference in a rotational behavior of the vehicle axle to be controlled relative to a further vehicle axle. The target differential slip is dependent on at least one of the detected actual vehicle deceleration and the detected target vehicle deceleration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. §371 of International Application No. PCT/EP2016/000668 filed on Apr. 26, 2016, and claims benefit to German Patent Application No. DE 10 2015 006 703.6 filed on May 23, 2015. The International Application was published in German on Dec. 1, 2016 as WO 2016/188603 A1 under PCT Article 21(2).

FIELD

The invention concerns a method for electronically controlling a vehicle deceleration in an ABS brake system, in particular in a utility vehicle, by adjusting braking pressures on a vehicle axle to be controlled depending on a detected actual differential slip and a device for carrying out the method.

BACKGROUND

In utility vehicles, a braking pressure controller that limits the braking pressure (electronic brake force limiter, EBL) is used in particular to distribute the desired braking effect of a brake system or a driver to the vehicle axles in a suitable manner, so that the vehicle is decelerated according to a driver's wishes or a target vehicle deceleration specified by a driver and at the same time remains stable depending on the driving situation. This is because in the event of hard braking, i.e. in the case of a high target vehicle deceleration, the weight of the vehicle is displaced to the front axle, so that the rear wheels on the rear axle are pressed less strongly onto the ground, whereby locking of the rear wheels is assisted.

By limiting the braking pressure on wheel brakes of the rear wheels, premature locking of the rear wheels during braking can be avoided. The limiting of the braking pressure can be carried out depending on an axle loading, because in the case of vehicles that are heavily loaded more braking force can be applied to the road by means of the rear axle than with unladen vehicles, in which excessive braking of the rear wheels amplifies the effect described above still further; the tendency to locking of the rear wheels increases.

For measuring the load in the case of spring suspension vehicles, a mechanical displacement sensor can be provided that is deflected to a greater or lesser extent depending on the load. In the case of pneumatic suspension vehicles, a pressure sensor that measures the pressure in a supporting bellows can be used for this. Depending on the load-dependent measurement signal, the weight can be concluded and limiting of the braking pressure can be carried out depending thereon.

In vehicles without such a mechanical displacement sensor or pressure sensor, in which no direct measurement of the load is provided, electronic braking pressure limiting can be provided. In this case, control of the braking pressure on the wheel brakes of the rear wheels is carried out depending on a difference between a wheel revolution rate of the front wheels and the wheel revolution rate of the rear wheels, or depending on the difference between a brake slip of the front wheels and the brake slip of the rear wheels. Because the difference in the wheel revolution rates or the brake slips is load-dependent, the braking pressure is distributed accordingly to the vehicle axles to prevent the wheels of the rear axle from prematurely showing a tendency to lock and in order to decelerate the vehicle as desired.

DE 10 2008 009 948 B3 discloses an electronic braking pressure controller, with which the braking pressure on the wheel brakes is controlled depending on an axle load sensor and the target vehicle deceleration. In the event of a failure of the axle load sensor, control of the braking pressure is carried out depending on the difference of the brake slips on the vehicle axles, i.e. depending on a measured actual differential slip.

DE 10 2011 118 130 B4 also discloses an electronic braking pressure controller, with which the braking forces are distributed depending on a determined actual differential slip or a determined speed difference of the wheels on the vehicle axles in order to decelerate the vehicle as desired. During this, the braking pressure on the wheel brakes of the rear wheels is limited if the actual differential slip exceeds a target differential slip. In this case, the target differential slip is a function of the target vehicle deceleration, wherein the target differential slip increases at higher target vehicle decelerations in order to guarantee a more sensitive transition to ABS control, which is more likely to intervene in this case rather than in the case of a constant target brake slip. It is disadvantageous in this case that an increase in the target differential slip in the case of a high target vehicle deceleration causes early actuation of the ABS controller, whereby the braking behavior is less smooth and less safe, because ABS control must intervene sooner.

In DE 10 2006 045 317 A1, an electronic braking force distribution is disclosed, with which the braking forces are distributed to the vehicle axles as a function of the vehicle deceleration instead of as a function of the differential slip. For this purpose, it is provided to measure an actual vehicle deceleration and to control the braking pressure on the rear axle on reaching a maximum deceleration, i.e. a defined threshold value for the actual vehicle deceleration. In this case, the maximum deceleration is defined in such a way that from said value the rear wheels tend to lock more than the front wheels. Thus, a tendency to locking of the rear wheels is matched to the tendency to locking of the front wheels. After exceeding the maximum deceleration, it is provided to build up the braking pressure on the wheel brakes of the rear wheels in a controlled manner, wherein the gradient is smaller than the gradient of the braking pressure that is building up on the wheel brakes of the front wheels. As a result, the braking pressure on the wheel brakes of the rear axle tracks the braking pressure on the wheel brakes of the front wheels, i.e. in order to remain lower by a defined value. It is disadvantageous in this case that above the maximum deceleration the braking behavior acts in an unsettled manner overall owing to the adjustment and is perceived as unpleasant.

SUMMARY

In an embodiment, the present invention provides a method for controlling a vehicle deceleration depending on a differential slip between two vehicle axles in a vehicle with an ABS brake system. The method includes detecting at least one of a target vehicle deceleration specified by the driver and an actual vehicle deceleration; and controlling a braking pressure on wheel brakes of a vehicle axle to be controlled by actuation of ABS brake valves in such a way that the braking pressure on the wheel brakes of the vehicle axle to be controlled is controlled depending on a detected actual differential slip, so that the actual differential slip corresponds to a target differential slip. The actual differential slip indicates a difference in a rotational behavior of the vehicle axle to be controlled relative to a further vehicle axle. The target differential slip is dependent on at least one of the detected actual vehicle deceleration and the detected target vehicle deceleration. The braking pressure on the wheel brakes of the vehicle axle to be controlled is limited if the vehicle axle to be controlled is over braked relative to the further vehicle axle. The target differential slip is reduced when increasing the actual vehicle deceleration and/or increasing the target vehicle deceleration towards synchronous wheel rotational behavior of the wheels of the vehicle axle to be controlled and the wheels of the further vehicle axle, so that the braking pressure on the vehicle axle to be controlled is already limited for a lower relative over braking of the vehicle axle to be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

FIG. 1 shows a vehicle with an electronic brake system as a block diagram;

FIG. 2 shows a graph of the dependency of an actual vehicle deceleration on a target vehicle deceleration to determine the control behavior;

FIGS. 3 a, b, c show graphs of a dependency of a target differential slip on an actual vehicle deceleration for use in differential slip control;

FIGS. 4a, b show a dependency of a controlled braking pressure on the vehicle axles depending on a load;

FIG. 5 shows a flow chart for carrying out the electronic vehicle deceleration control according to an embodiment of the invention;

FIG. 6 shows a flow chart for carrying out the differential slip control as part of the electronic vehicle deceleration control according to FIG. 5; and

FIGS. 7a, b show the profile of the actual vehicle deceleration when carrying out the vehicle deceleration control during a braking process and during a slowing down process of the vehicle.

DETAILED DESCRIPTION

Embodiments of the invention provide methods and devices for electronically controlling a vehicle deceleration depending on a differential slip between two vehicle axles in an ABS brake system that guarantees pleasant and safe braking with smooth braking behavior.

Embodiments of the invention provide for adjusting a braking pressure on wheel brakes of a vehicle axle that is to be controlled depending on a detected actual differential slip by limiting, preferably maintaining, the braking pressure on the wheel brakes of the vehicle axle to be controlled, in order to prevent over braking of the vehicle axle to be controlled relative to a further vehicle axle. In this case, the actual differential slip is controlled to a target differential slip, wherein the target differential slip is a function of a detected actual vehicle deceleration and/or a detected target vehicle deceleration. According to embodiments of the invention, the target differential slip decreases with increasing actual vehicle deceleration or target vehicle deceleration towards synchronous wheel rotational behavior of the wheels of the vehicle axle to be controlled and the further vehicle axle, such that premature locking of the vehicle axle to be controlled is counteracted by methods according to embodiments of the invention; the braking pressure on the wheel brakes of the vehicle axle to be controlled is thus already limited for a small relative over braking of the vehicle axle to be controlled; safety and braking comfort increase.

In this case, the actual differential slip indicates a difference between a current rotational behavior of the vehicle axle to be controlled, for example a rear axle of the vehicle, and the further vehicle axle, for example a front axle of the vehicle. The rotational behavior is characterized in this case by a brake slip or a rotation rate of the wheels of the corresponding vehicle axle. The actual differential slip thus indicates the extent to which the vehicle axle to be controlled is under braked or over braked in relation to the further vehicle axle. In this case, over braking means that the vehicle axle to be controlled is turning slower relative to the further vehicle axle, i.e. in particular has a greater brake slip, whereas under braking means that the vehicle axle to be controlled has a smaller brake slip than the further vehicle axle, i.e. is turning faster.

In order to adjust the actual differential slip to the target differential slip, the actual differential slip and the target differential slip are compared and the rear axle braking pressure is adjusted accordingly in the case of a deviation, wherein the rear axle braking pressure is limited in the event of over braking, preferably being maintained and possibly even reduced, and is increased in the event of under braking. As a result, the tendency to locking of the vehicle axle to be controlled can be matched to the tendency to locking of the further vehicle axle.

The actual differential slip is preferably proportional to a difference of the brake slip of the rear axle that is driven by an engine minus the brake slip of the front axle, preferably normalized to the brake slip of the driven rear axle. However, the method of calculation is not limited to this; for example, depending on the rotation rate of the vehicle axles, the actual differential slip can also be determined from the difference of the rotation rate of the front axle minus the rotation rate of the driven rear axle, preferably normalized to the rotation rate of the front axle. Other methods of calculation that characterize the difference in the rotational behavior are also conceivable.

In order to take into account an engine drag torque, wear on the wheels and possibly highway properties, at the start of braking, i.e. when no braking forces are yet acting, an offset differential slip can be determined that can be taken into account during the subsequent control of the braking pressures depending on the differential slip. According to the above method of calculation, the offset differential slip is greater than zero in this case, because the driven rear axle is turning more slowly than the rotating front axle owing to an engine drag torque. The target differential slip thus decreases to zero starting from the offset differential slip, i.e. towards synchronous wheel rotational behavior of the wheels of the vehicle axle to be controlled and the further vehicle axle.

If the vehicle axle to be controlled is a rear axle that can be loaded and the further vehicle axle is the front axle of the vehicle, as a result the advantage can be achieved that the rear axle braking pressure is already limited in the case of a lower actual differential slip. In this case, it is advantageously taken into account that in the case of a high actual vehicle deceleration or a high target vehicle deceleration, the rear axle that can be loaded tends to slip more readily, i.e. is over braked, and thus according to the above method of calculation there tends to be higher actual differential slip, which is further increased in the unladen state.

If according to the above method of calculation the actual differential slip is greater than the target differential slip here, the rear axle is over braked relative to the front axle, i.e. the rear axle braking pressure is limited. If on the other hand the actual differential slip is less than the target differential slip, the rear axle is under braked relative to the front axle, i.e. the rear axle braking pressure is increased. Thus, the rear axle braking pressure is already limited owing to the target differential slip decreasing with increasing actual vehicle deceleration or target vehicle deceleration in the case of lower actual differential slip, i.e. in the case of lower relative over braking; premature intervention by the overlaid ABS control can be prevented; the driving behavior is thereby smoother and safer overall.

If alternatively the front axle is the vehicle axle that is to be controlled and the further vehicle axle is the rear axle, for example in the case of a utility vehicle with a greater load on the rear axle, for example a bus with the engine over the rear axle, the front axle is pressed less heavily onto the road and thus tends more to start to slip, wherein as a result according to the above method of calculation the actual differential slip tends to be smaller starting from the offset differential slip. If in this case the actual differential slip determined according to the above method of calculation is greater than the target differential slip, the front axle is under braked relative to the rear axle, i.e. the front axle braking pressure is increased. If on the other hand the actual differential slip is smaller than the target differential slip, the front axle is over braked relative to the rear axle, i.e. the rear axle braking pressure is limited. The target differential slip decreasing towards zero from the offset differential slip thus ensures that premature intervention of the overlaid ABS control on the front axle can be avoided by already limiting the braking pressure at lower relative over braking of the front axle; the driving behavior is thus smoother and safer overall.

The target differential slip can be dependent on the actual vehicle deceleration and at the same time also dependent on the target vehicle deceleration, wherein for the case in which the actual vehicle deceleration is less than the target vehicle deceleration, i.e. the braking demand desired by the driver cannot be fully met, the target differential slip decreases starting from the offset differential slip only in the case of a higher value of the actual vehicle deceleration, in contrast to the case in which the actual vehicle deceleration corresponds to the target vehicle deceleration within a tolerance of for example 10%. If on the other hand the actual vehicle deceleration is greater than the target vehicle deceleration, i.e. the vehicle is braked too heavily in respect of the driver's wishes, the target differential slip is already decreasing at a lower actual vehicle deceleration, in contrast to the case in which the actual vehicle deceleration corresponds to the target vehicle deceleration within the tolerance. As a result, the braking pressure is only increased in the case of excessively heavy braking at a higher actual vehicle deceleration, and in the case of insufficiently heavy braking is already increased at a lower actual vehicle deceleration. As a result, the level of safety in the vehicle can advantageously be increased.

The actual vehicle deceleration is for example determined by an acceleration sensor or from the derivative of a vehicle reference speed that is determined by means of wheel revolution rate sensors. In this case, it can be taken into account whether the vehicle is already decelerating without actuation of the brakes, for example by rolling resistance and air resistance, a highway gradient or a retarder in the vehicle. In this case, the method is based on a difference of a measured actual vehicle deceleration and a corresponding offset vehicle deceleration.

Furthermore, a tolerance can preferably be provided that lies above and/or below the target differential slip. As a result, a minimum actuation time or pulse duration of brake valves for adjusting the braking pressures in the brake system can advantageously be taken into account. This is because the resolution for controlling the braking pressures or for controlling the actual differential slip to the target differential slip is limited by the actuation time of the brake valves. This is in particular advantageous in the case of unclocked brake valves. In the case of clocked brake valves, the tolerance can be omitted. Clocked means that a solenoid valve is a brake valve, inlet valve and/or outlet valve that is energized and deenergized with defined clock times, i.e. for example in the case of an inlet valve is alternately closed and opened, in order for example to increase the braking pressure on the wheel brake to be controlled and to control the actual differential slip to the target differential slip. In the case of unclocked brake valves on the other hand, a continuous build-up of pressure is carried out by deenergizing, i.e. opening the brake valve continuously until the target differential slip is reached.

According to one embodiment, besides the differential slip control a further vehicle deceleration control can be used that additionally controls the braking pressure on the wheel brakes of the vehicle axles depending on the detected actual vehicle deceleration, i.e. the negative actual acceleration of the vehicle, as follows:

In this case, in the following a first vehicle axle corresponds to the further vehicle axle described above, for example the front axle, and a second vehicle axle corresponds to the vehicle axle to be controlled that was described above, for example the rear axle.

The vehicle deceleration control has the effect that the braking pressure on the wheel brakes of the first and the second vehicle axles is limited if the actual vehicle deceleration exceeds a maximum deceleration, and the braking pressure on the wheel brakes of the first and the second vehicle axles is increased if the actual vehicle deceleration is less than a minimum deceleration. In this case, limiting means that the braking pressure on the wheel brakes of the two vehicle axles to be controlled may not be increased further, i.e. the braking pressure can be maintained at the current value or even reduced to a lower value depending on the driver's wishes.

A method for controlling the vehicle deceleration (vehicle deceleration control) by adjusting the braking pressures on the wheel brakes of the first and the second vehicle axles is thus advantageously provided, wherein control intervention is made in the first and the second vehicle axles, taking into account the maximum and the minimum deceleration, and in the second vehicle axle additionally taking into account the actual differential slip, wherein the actual differential slip preferably results from the difference in the rotational behavior of the second vehicle axle and the first vehicle axle. As a result, the control behavior in the vehicle can advantageously be smoothed, because after reaching the maximum deceleration further control interventions, for example by the differential slip controller, are inhibited.

The differential slip control according to said embodiment is subject to the requirement that the braking pressure on the wheel brakes of the second vehicle axle is limited if the actual vehicle deceleration is greater than the maximum deceleration, for example owing to the differential slip control, and is increased if it is lower than the minimum deceleration. In the case in which the actual vehicle deceleration is lower than the minimum deceleration, the differential slip control is interrupted in a pulsed manner; control depending on the actual differential slip is then no longer enabled for safety reasons. Limiting the braking pressure on exceeding the maximum deceleration by the actual vehicle deceleration is advantageously used for smoothing of the deceleration profile as fewer control interventions occur. This is in particular the case if the braking pressure is also not reduced during the limiting, i.e. is maintained by the differential slip control.

Advantageously however, the braking pressure on the second vehicle axle can also be reduced by the differential slip control if the actual vehicle deceleration is greater than the maximum deceleration and the second vehicle axle is over braked. Owing to this, the actual vehicle deceleration advantageously reduces faster to the maximum deceleration and the over braking of the second vehicle axle decreases at the same time; furthermore, safety and braking comfort increase.

The maximum deceleration and the minimum deceleration are in particular dependent upon a target vehicle deceleration that is specified by the driver, so that a maximum deceleration characteristic and a minimum deceleration characteristic result. In this case, the target vehicle deceleration can for example be determined by a braking value transducer, i.e. a displacement sensor, that detects the actuation travel of a brake valve or a brake pedal actuated by the driver, or a pressure sensor that detects a braking pressure adjusted by the driver with a brake valve. From the characteristic curves, in the case of a detected target vehicle deceleration the value of the maximum deceleration or the minimum deceleration can be determined and can form the basis for the subsequent vehicle deceleration control. During this, the maximum deceleration characteristic profile is constant or advantageously harmonic, i.e. rises smoothly without steps, depending on the target vehicle deceleration, for example.

As a result, a pleasant brake feel and improved braking pressure smoothness can advantageously be achieved. This is because by limiting the braking pressure on the wheel brakes of the first and second vehicle axles to the harmonically varying maximum deceleration characteristic depending on the target vehicle deceleration, which is also referred to as the Feeling graph, the braking force is limited on the two vehicle axles to be controlled in such a way that an actual vehicle deceleration resulting therefrom is perceived by the driver as pleasant. The profile of the Feeling graph also enables improved braking pressure smoothness, because the braking pressure can be controlled more sensitively depending on the target vehicle deceleration, i.e. on the driver's wishes.

If the actual vehicle deceleration is greater than or equal to the maximum deceleration and the driver increases the target vehicle deceleration by an additional operation of the brake pedal or a brake valve, i.e. a braking process is occurring, a stronger deceleration of the vehicle only then takes place if there is a further vehicle axle that is not controlled by the method, the wheel brakes of which are then actuated accordingly, unless the actual vehicle deceleration is again less than the maximum deceleration owing to this. If the actual vehicle deceleration is greater than or equal to the maximum deceleration and the driver reduces the target vehicle deceleration, for example by releasing the brake pedal, i.e. there is a slowing down process, the braking pressure can advantageously also be reduced to control the actual vehicle deceleration faster to the maximum deceleration.

The maximum deceleration is determined depending on the target vehicle deceleration, in particular in such a way braking by the two vehicle axles to be controlled that is effectively unnecessary can be avoided by limiting the braking pressure of said vehicle axles to a value that is the maximum required to decelerate the vehicle according to the target vehicle deceleration specified by the driver. In this case, in particular the braking effect that can be achieved by the uncontrolled further vehicle axles that may be in the vehicle is taken into account.

The maximum deceleration characteristic is preferably extended by an upper tolerance and a lower tolerance to advantageously take into account a minimum actuation time or pulse duration of the brake valves in the brake system. This is because triggering the control of the braking pressures or control of the actual vehicle deceleration to the maximum deceleration characteristic is limited by the actuation time of the brake valves.

In order to guarantee pleasant and safe braking, even in the case of different load states of the vehicle, the maximum deceleration is preferably load-dependent and dependent on the target vehicle deceleration. Therefore, the maximum deceleration characteristic profile is flatter for a heavy load and steeper for a light load, i.e. rises faster, so that the braking pressure on the wheel brakes of the two vehicle axles to be controlled for higher load is already maintained for a lower actual vehicle deceleration than for a lighter load. As a result, the driver can advantageously be informed of the load state of the vehicle by the controlled braking effect, since the vehicle is decelerated faster with a light load than for a high load for the same target vehicle deceleration.

If the actual vehicle deceleration is less than the minimum deceleration, braking force limiting on the two vehicle axles to be controlled is advantageously terminated or blocked in order to ensure that a minimum deceleration is not prevented, for example in order to satisfy requirements regarding a legally required minimum braking effect depending on the target vehicle deceleration; which increases safety.

The braking pressure on the two vehicle axles to be controlled can both be increased and also maintained by the vehicle deceleration control if the actual vehicle deceleration is less than the maximum deceleration and greater than the minimum deceleration. On the first vehicle axle, the braking pressure can additionally also be reduced.

In said region, during a braking process the braking pressure on the wheel brakes of the first vehicle axle is preferably only maintained or increased, and during a slowing down process is only maintained or reduced, in order to advantageously achieve slowing down of the vehicle with only a few control interventions. In this case, the term braking process means that the actual vehicle deceleration increases, and the term slowing down process means that the actual vehicle deceleration decreases. In addition, it can be provided that the braking pressure during a braking process is only maintained or increased if the driver also increases the target vehicle deceleration by a harder operation of the brake pedal or the brake valve and during a slowing down process is only maintained or reduced if the driver also releases the brake pedal and thereby actively reduces the target vehicle deceleration. This has a positive effect on the brake feel of the driver, because he receives a corresponding response from the vehicle when operating the brake pedal and the adjustment of the braking pressure is not even carried out if the actual vehicle deceleration increases or decreases with a varying gradient, for example. In addition, it can also be provided that initial over braking during a braking process can be compensated by a pressure reduction and initial under braking can be compensated during a slowing down process by increasing the pressure.

In order to improve the smoothness of the braking pressure, during a transition from a braking process to a slowing down process or from a slowing down process to a braking process, a time period for reducing or increasing the braking pressures on the wheel brakes of the two vehicle axles to be controlled can be extended in order in particular to overcome hysteresis behavior during the reversal of the direction of the braking effect on the wheel brakes.

For controlling the vehicle deceleration by adjusting the corresponding braking pressures on the wheel brakes of the two vehicle axles to be controlled, a device according to an embodiment of the invention, for example a VD control device (vehicle deceleration control device), controls ABS brake valves, i.e. inlet valves implemented as 2/2-solenoid valves, and outlet valves on the wheel brakes of the two vehicle axles to be controlled in such a way that a braking pressure on the vehicle axle to be controlled is either maintained or increased. In addition, the VD control device can also optionally reduce the braking pressure on both vehicle axles to be controlled by venting in order to remove or reduce the braking force.

In this case, the ABS brake valves are part of an anti-lock brake system (ABS) present in the vehicle to control the brake slip on the individual wheels. The ABS brake valves can be actuated both by an ABS control device and also by the VD control device. However, the ABS control device has priority over the VD control device, so that on detecting an excessive brake slip on one of the wheels of the two vehicle axles to be controlled, conventional ABS control is carried out by the ABS control device and the vehicle deceleration control by the VD control device is inhibited.

Various load-dependent maximum deceleration characteristics, on which the vehicle deceleration control is based depending on the load on the vehicle, can preferably be stored in the VD control device. The load state, depending on which the maximum deceleration characteristic is selected, can for example be derived from the differential slip control described above. If the vehicle is not loaded, i.e. a low weight force is acting on the rear axle for example, the rear wheels tend more to lock for a given brake actuation or for a predetermined target vehicle deceleration than for a fully loaded vehicle with greater weight on the rear axle. The actual differential slip thus varies depending on the load of the vehicle for a predetermined target vehicle deceleration. A load on the vehicle can thus be concluded from the actual differential slip, and distribution of the braking forces on the vehicle axles depending on the load can be achieved indirectly by the differential slip control.

However, it is also possible to use an arbitrary axle load sensor to determine the load state, which measures either a displacement between for example a frame of the vehicle and the loaded vehicle axle or a pressure acting on the vehicle axle that can be loaded or a supporting bellows on the vehicle axle that can be loaded by means of a pressure sensor, from which a load state can be concluded by the VD control device.

However, the load information can also be transmitted via a CAN bus from an ECAS system to the VD control device, which controls the pneumatic suspension of the vehicle electronically depending on an axle loading. Moreover, the VD control device can determine a current mass of the vehicle from information about the current engine power and the resulting vehicle acceleration, from which a load can be concluded with knowledge of the unladen mass of the vehicle. Furthermore, the presence of a trailer can also be taken into account during the determination of the load or during the selection of the maximum deceleration characteristic, wherein said information can be provided to the VD control device via the CAN bus by a trailer control valve for example.

FIG. 1 shows a vehicle 200 with a brake system 100 with the components that are relevant here. Therefore, a rear axle HA with rear wheels 1, 2 is provided as the axle to be controlled or the second vehicle axle and a front axle VA with front wheels 3, 4 is provided as a further or first vehicle axle. The rear wheels 1, 2 can be braked by means of rear wheel brakes 5, 6 and the front wheels 3, 4 can be braked by means of front wheel brakes 7, 8 by transferring a braking pressure p provided by means of brake cylinders 9.1 via a dual circuit brake valve 9.2 to the rear wheel brakes 5, 6 or the front wheel brakes 7, 8 depending on a target vehicle deceleration z_(Soll) that is specified by the driver. In this case, the target vehicle deceleration z_(Soll) can for example be detected by a pressure sensor 9.3 acting as a braking value transducer that detects a braking pressure controlled by the driver via the brake valve 9.2, according to FIG. 1 by way of example a front axle braking pressure p_(VA). The brake system 100 can be an electrohydraulic or electropneumatic brake system, i.e. a liquid pressure or an air pressure is controlled. The brake system 100 is electrically controlled, in particular by an EBS control device 101.

For brake slip-controlled deceleration, the brake system 100 comprises an anti-lock brake system (ABS) with an ABS control device 10 that actuates ABS brake valves 11, 12, 13, 14, 21, 22, 23, 24 in a pulsed manner depending on a detected brake slip. According to said embodiment, the ABS control device 10 is integrated within the EBS control device 101.

The ABS brake valves 11, 12, 13, 14 on the rear wheel brakes 5, 6 and the ABS brake valves 21, 22, 23, 24 on the front wheel brakes 7, 8 are each implemented as 2/2-solenoid valves, wherein an inlet valve 11, 13, 21, 23 and an outlet valve 12, 14, 22, 24 are provided for each wheel brake 5, 6, 7, 8. If the inlet valves 11, 13 are opened and the outlet valves 12, 14 are shut off, the rear wheel brakes 5, 6 are subjected to a rear axle braking pressure p_(HA) controlled by the brake valve 9.2 and thus corresponding braking is effected by the rear axle HA. If on the other hand the inlet valves 11, 13 are shut off and the outlet valves 12, 14 are opened, the rear wheel brakes 5, 6 are vented and thus the braking effect by the rear axle HA is reduced. If both the inlet valves 11, 13 and the outlet valves 12, 14 are shut off, a rear axle braking pressure p_(HA) acting on the rear wheel brakes 5, 6 is maintained. The same applies to the ABS brake valves 21, 22, 23, 24 on the front wheel brakes 7, 8, by the adjustment of which the front axle braking pressure p_(VA) is controlled.

In the context of the exemplary embodiment, there is no distinction between the left and the right rear wheel brake 5, 6 in relation to the braking pressure or the braking effect, i.e. both rear wheel brakes 5, 6 are subjected to the same rear axle braking pressure p_(HA) in order to achieve a braking effect corresponding to the target vehicle deceleration z_(Soll) together with the front wheel brakes 7, 8, on each of which the front axle braking pressure p_(VA) acts. It is however also possible to control the ABS brake valves 11, 12, 13, 14, 21, 22, 23, 24 of the individual wheel brakes 5, 6, 7, 8 independently of each other, so that there can be different braking pressures on each of the wheel brake 5, 6, 7, 8.

According to embodiments of the invention, in addition to the ABS control, electronic control of the vehicle deceleration (vehicle deceleration control) is also provided that controls the front axle braking pressure p_(VA) and the rear axle braking pressure p_(HA) in such a way that a determined actual vehicle deceleration z_(Ist) that is dependent on the target vehicle deceleration z_(Soll), i.e. a negative acceleration, is set in the vehicle 200. In this case, the rear axle braking pressure p_(HA) is additionally controlled depending on a detected actual differential slip DS_(Ist).

The vehicle deceleration control is controlled by a VD control device 15, which according to said embodiment is integrated within the EBS control device 101 and is implemented to actuate the ABS brake valves 11, 12, 13, 14 on the rear wheel brakes 5, 6 and the ABS brake valves 21, 22, 23, 24 on the front wheel brakes 7, 8. However, the ABS control device 10 has priority over the VD control device 15 regarding the control of the ABS brake valves 11, 12, 13, 14, 21, 22, 23, 24. I.e., on detecting brake slip that is not allowed on one of the rear wheels 1, 2 or on one of the front wheels 3, 4, vehicle deceleration control that has already started is terminated and conventional ABS control is carried out by the ABS control device 10.

According to said embodiment, the vehicle deceleration control determines as follows at which actual vehicle decelerations z_(Ist) the ABS brake valves 11, 12, 13, 14, 21, 22, 23, 24 are actuated, and as a result the vehicle 200 is decelerated depending on the target vehicle deceleration z_(Soll):

A maximum deceleration characteristic z_(Kmax) and a minimum deceleration characteristic z_(Kmin) as functions of the target vehicle deceleration z_(Soll) are stored in the VD control device 15, each of which constitutes determined dependencies of a maximum deceleration z_(max) or a minimum deceleration z_(min) on the target vehicle deceleration z_(Soll). The maximum deceleration characteristic z_(Kmax) can also be referred to as a Feeling graph that varies harmonically and as a result produces a pleasant brake feel for the driver.

According to FIG. 2, the target vehicle deceleration z_(Soll) is plotted in a graph by way of example in percentage of the actuation of the brake valve 9.2 against the actual vehicle deceleration z_(Ist) in m/s². The maximum deceleration characteristic z_(Kmax) and the minimum deceleration characteristic z_(Kmin) divide the diagram into three regions B1, B2, B3. A first region B1 lies between the x-axis (z_(Soll)) and the minimum deceleration characteristic z_(Kmin), a second region B2 lies between the minimum deceleration characteristic Z_(Kmin) and the maximum deceleration characteristic z_(Kmax) and a third region B3 lies above the maximum deceleration characteristic z_(Kmax).

Said three regions B1, B2, B3 determine how the VD control device 15 can actuate the ABS brake valves 11, 12, 13, 14 on the rear wheel brakes 5, 6 and the ABS brake valves 21, 22, 23, 24 on the front wheel brakes 7, 8 and thereby the braking force or the rear axle braking pressure p_(HA) or front axle braking pressure p_(VA) that is controlled on the rear wheel brakes 5, 6 or front wheel brakes 7, 8 depending on the target vehicle deceleration z_(Soll) and the actual vehicle deceleration z_(Ist). The rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) are controlled depending on the three regions B1, B2, B3 as follows:

If a determined braking pressure p_(HA), p_(VA) is controlled at the wheel brakes 5, 6, 7, 8 by means of the brake valve 9.2 and thereby a target vehicle deceleration z_(Soll) is demanded by the driver and the resulting actual vehicle deceleration z_(Ist), which results from the braking of the front and the rear wheel brakes 5, 6, 7, 8, lies in the first region B1, the VD control device 15 causes opening of the inlet valves 11, 13, 21, 23 and closing of the outlet valves 12, 14, 22, 24 so that a build-up of pressure can be carried out both on the rear wheel brakes 5,6 and the front wheel brakes 7, 8 and an increasing deceleration by the rear wheels 1, 2 and the front wheels 3, 4 is enabled thereby.

The rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) are thus increased if the actual vehicle deceleration z_(Ist) for a defined predetermined target vehicle deceleration z_(Soll) is less than the minimum deceleration z_(min) resulting from the minimum deceleration characteristic z_(Kmin) for the target vehicle deceleration z_(Soll). According to FIG. 2, by way of example it is indicated that a minimum deceleration z_(min) and a maximum deceleration z_(max) result from the y-values of the characteristic curves z_(Kmin) and z_(Kmax) for a defined target vehicle deceleration z_(Soll) (x-value).

Said first region B1 is determined by the minimum deceleration characteristic z_(Kmin), which for example can specify a legally specified minimum deceleration depending on the target vehicle deceleration z_(Soll). I.e., below the minimum deceleration characteristic z_(Kmin) a deceleration is forced for safety reasons—unless the higher-level ABS control intervenes.

If the actual vehicle deceleration z_(Ist) for a defined target vehicle deceleration z_(Soll) lies in the third region B3, i.e. the actual vehicle deceleration z_(Ist) is greater than the maximum deceleration z_(max) that results from the maximum deceleration characteristic z_(Kmax) for the target vehicle deceleration z_(Soll), the inlet valves 11, 13, 21, 23 and the outlet valves 12, 14, 22, 24 are actuated so that the rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) are limited.

Thus, it is not enabled that the rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) continue to increase if the target vehicle deceleration z_(Soll) specified by the driver is increased; unless the actual vehicle deceleration z_(Ist) is displaced into the first or second region B1, B2 again by the increasing target vehicle deceleration z_(Soll). In the third region B3, the vehicle 200 can also only be more strongly decelerated by a further wheel brake of a vehicle axle that is not controlled, for example a second rear axle, in the braking behavior of which the vehicle deceleration control does not intervene.

The rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) and thus the braking force of the rear and front wheel brakes 5, 6, 7, 8 are also limited to a defined value, which is predetermined by the maximum deceleration characteristic z_(Kmax), depending on the actual vehicle deceleration z_(Ist). By controlling to said harmonically varying maximum deceleration characteristic z_(Kmax), a pleasant brake feel for the driver can be achieved.

If the actual vehicle deceleration z_(Ist) lies between the maximum deceleration characteristic z_(Kmax) and the minimum deceleration characteristic z_(Kmin), i.e. in the second region B2, an increase, reduction and also maintenance of the rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) are enabled to decelerate the vehicle 200.

In this case, in the second region B2 the actual vehicle deceleration z_(Ist) is additionally controlled depending on the actual differential slip DS_(Ist) by maintaining, reducing or increasing the rear axle braking pressure p_(HA) depending on the level of an actually existing actual differential slip DS_(Ist). Thus, a differential slip control (DSR) is carried out.

According to the embodiment, differential slip means the difference between the rotational behavior of the front wheels 3, 4 and the rotational behavior of the rear wheels 1, 2. According to said example, the rotational behavior is characterized by a front axle brake slip DS_(VA) or a rear axle brake slip DS_(HA), both of which result from the average value of the brake slips of the individual wheels 1, 2, 3, 4 of the respective vehicle axle VA, HA.

According to said embodiment, the actual differential slip DS_(Ist) is formed from the front axle brake slip DS_(VA) and the rear axle brake slip DS_(HA) as follows:

DS_(Ist)˜(DS_(HA)−DS_(VA))/DS_(HA)*100%;   (1.1)

the difference in the rotational behavior of the vehicle axles VA, HA is thus defined by a deviation of the front axle brake slip DS_(VA) from the rear axle brake slip DS_(HA), which is in particular increased if the rear axle HA is over braked in contrast to the front axle VA, i.e. tends more to slip.

An alternative method of calculation of the rotational behavior of the rear wheels 1, 2 relative to the rotational behavior of the front wheels 3, 4 is to determine the actual differential slip DS_(Ist) from the revolution rates v_(VA) or v_(HA) of the vehicle axles VA, HA, i.e.

DS_(Ist)(v _(VA) −v _(HA))/v _(VA)*100%,   (1.2)

wherein the revolution rates v_(VA) or v_(HA) for example can result from an average value of the revolution rates of the wheels 1, 2, 3, 4 of the respective vehicle axles VA, HA.

The aforementioned two equations 1.1 and 1.2 are only used by way of example to obtain a variable characterizing the difference in the rotational behavior. However, any other dependencies can also be used.

For the differential slip control, it is provided that the VD control device 15 determines the actual differential slip DS_(Ist) continuously following actuation of the brake valve 9.2 and in the presence of the target vehicle deceleration z_(Soll). If the actual brake slip DS_(Ist) deviates from a target brake slip DS_(Soll), wherein in the case of unclocked ABS brake valves 11, 12, 13, 14 a predetermined tolerance value DS_(T) can be incorporated, and the actual vehicle deceleration z_(Ist) lies in the second region B2, then the VD control device 15 controls the ABS brake valves 11, 12, 13, 14 of the rear wheels 1, 2 so that the actual differential slip DS_(Ist) again corresponds to the target differential slip DS_(Soll) within the tolerance DS_(T):

If the actual differential slip DS_(Ist) lies below the target differential slip DS_(Soll), the rear axle HA is under braked in relation to the target differential slip DS_(Soll) relative to the front axle VA. In this case, the ABS inlet valves 11, 13 are opened so that the rear axle braking pressure p_(HA) is increased. If the actual differential slip DS_(Ist) lies above the target differential slip DS_(Soll), the rear axle brake slip DS_(HA) in relation to the target differential slip DS_(Soll) is greater than the front axle brake slip DS_(VA), i.e. the rear axle HA is over braked. In this case, the rear axle braking pressure p_(HA) is limited, preferably maintained, so that the braking effect on the rear wheels 1, 2 is not increased further.

The target differential slip DS_(soll) to which the slip is controlled is, according to said embodiment, dependent on the actual vehicle deceleration z_(Ist), as shown in FIG. 3a . For low actual vehicle decelerations z_(Ist), the target differential slip DS_(soll) is initially constant and at a determined offset differential slip DS_(Off). According to said embodiment, the offset differential slip DS_(Off) is determined at the start of braking, i.e. when no braking forces are yet acting on the wheel brakes 5, 6, 7, 8, so that in particular the engine drag torque and the wear on the wheels 1, 2, 3, 4 are taken into account. The offset differential slip DS_(Off) is positive during a calculation of the actual differential slip DS_(Ist) according to the equations 1.1 or 1.2 if the rear axle HA is the axle being driven by the engine of the vehicle 100, because the rear axle HA is turning more slowly owing to the engine drag torque. Starting from the offset differential slip DS_(Off), the target differential slip DS_(Soll) reduces towards zero, i.e. towards synchronous wheel rotational behavior of the wheels 1, 2, 3, 4 of the first and second vehicle axles HA, VA.

If the actual differential slip, for example at the start of the differential slip control, lies above the target differential slip DS_(Soll) and also above the tolerance DS_(T) at the point DS₁, the rear axle braking pressure p_(HA) is initially limited. According to said exemplary embodiment, only a tolerance DS_(T) above the target differential slip DS_(Soll) is provided. Alternatively or additionally however, a tolerance DS_(T) below the target differential slip DS_(Soll) can also be provided. If a higher target vehicle deceleration z_(Soll) is demanded by the driver by further actuation of the brake valve 9.2, the vehicle 200 is additionally only still decelerated with the front wheel brakes 7, 8 if the actual vehicle deceleration z_(Ist) does not exit the second region B2; the actual vehicle deceleration z_(Ist) increases. As a result, the actual differential slip DS_(Ist) shifts from the point DS₁ to the point DS₂, for example exactly on the DS_(Soll) curve, so that the limiting of the rear axle braking pressure p_(HA) is removed and the rear axle braking pressure p_(HA) is increased in a pulsed manner. Owing to the additional braking effect of the rear wheel brakes 5, 6, in the further profile the braking of the point DS₂ now shifts towards increasing actual vehicle deceleration z_(Ist) again in the direction above the DS_(Soll) curve at the point DS₃. If the actual differential slip DS_(Ist) reaches or lies slightly above the tolerance value DS_(T), a further pressure increase on the rear wheel brakes 5, 6 is shut off, the rear axle braking pressure p_(HA) is again limited and only braking of the front wheel brakes 7, 8 is active.

In the case of increasing actual vehicle deceleration z_(Ist), the target differential slip DS_(Soll) reduces to smaller values. As a result, the circumstance is taken into account that the rear axle HA tends to be over braked in the case of high decelerations of the vehicle 200 and in particular a low or high load, a high centre of gravity of the load and a short wheelbase, i.e. it tends more to slip, i.e. the actual differential slip DS_(Ist) tends to shift to higher values during a calculation according to the equations 1.1 or 1.2. The aim of the method is thus to carry out braking force limiting on the rear axle HA for longer and to inhibit an increase in pressure for a higher deceleration to which the front axle VA contributes in the second region B2 without pressure limiting. Slipping of the rear axle HA and intervention by the higher-level ABS control that is caused as a result can be avoided and the braking is smoother and safer.

In a similar manner, for the case in which the first vehicle axle is the rear axle HA and the second vehicle axle is the front axle VA, for example if the method is carried out in a bus as the utility vehicle, and the actual differential slip DS_(Ist) is determined according to the equations 1.1 or 1.2, the target differential slip DS_(Soll) varies as shown in FIG. 3b . I.e., for increasing actual vehicle deceleration z_(Ist), target differential slip DS_(Soll) also decreases to smaller values, i.e. towards synchronous wheel rotational behavior of the rear wheels 1, 2 and the front wheels 3, 4, wherein in contrast to the inverse case the front axle braking pressure p_(VA) is limited if the actual differential slip DS_(Ist) is smaller than the target differential slip DS_(Soll) and the front axle braking pressure p_(VA) is increased if the actual differential slip DS_(Ist) is greater than the target differential slip DS_(Soll). In this case, the tolerance D_(T) runs below the DS_(Soll) curve.

If in this case the actual differential slip DS_(Ist), for example at the start of the differential slip control, is below the target differential slip DS_(Soll) and also below the tolerance DS_(T) at the point DS₁ in FIG. 3b , the front axle braking pressure p_(VA) is initially limited. If a higher target vehicle deceleration z_(Soll) is demanded by the driver by further actuation of the brake valve 9.2, the vehicle 200 is still only additionally decelerated with the rear wheel brakes 5, 6 if the actual vehicle deceleration z_(Ist) does not exit the second region B2; the actual vehicle deceleration z_(Ist) increases. As a result, the actual differential slip DS_(Ist) shifts from the point DS₁ to the point DS₂ above the DS_(Soll) curve, so that the limiting of the front axle braking pressure p_(VA) is removed and the front axle braking pressure p_(VA) is increased. Owing to the additional braking effect of the front wheel brakes 7, 8, in the further profile the braking of the point DS₂ now shifts towards increasing actual vehicle deceleration z_(Ist) again in the direction below the DS_(Soll) curve to the point DS₃, because the front axle VA is again relatively over braked owing to the increased front axle braking pressure p_(VA), whereby the actual differential slip DS_(Ist) decreases according to the equations 1.1 and 1.2. If the actual differential slip DS_(Ist) reaches or lies slightly below the tolerance value DS_(T), a further increase in pressure on the front wheel brakes 7, 8 is again prevented, the front axle braking pressure p_(VA) is again limited and an additional deceleration of the vehicle 200 is again only effected by the rear wheel brakes 5, 6.

It is thus achieved that in the case of high decelerations of the vehicle 200 and in particular a small or high load, a high centre of gravity of the load and a short wheelbase, the front axle VA, which tends to be over braked, tends more to slip, i.e. the actual differential slip DS_(Ist) tends to shift to smaller values during a calculation according to the equations 1.1 or 1.2, slipping of the front axle VA and a resulting intervention by the higher level ABS control can be avoided; the braking is smoother and safer.

According to FIG. 3c , the decreasing target differential slip DS_(Soll) shifts to lower or higher actual vehicle decelerations z_(Ist), depending on the actual vehicle deceleration z_(Ist) and the target vehicle deceleration z_(Soll). The target differential slip DS_(Soll) is thus in this case both dependent on the actual vehicle deceleration z_(Ist) and also on the target vehicle deceleration z_(Soll). If the actual vehicle deceleration z_(Ist) is less than the target vehicle deceleration z_(Soll), the target differential slip DS_(Soll) decreases, starting from the offset differential slip DS_(Off), only in the case of higher values of the actual vehicle deceleration z_(Ist), in contrast to the case in which the actual vehicle deceleration z_(Ist) corresponds to the target vehicle deceleration z_(Soll) within a tolerance of for example 10%, so that the profile represented by the dashed curve DS¹ _(Soll) in FIG. 3c results. If on the other hand the actual vehicle deceleration z_(Ist) is greater than the target vehicle deceleration z_(Soll), the target differential slip DS_(Soll) already decreases according to the dashed curve DS² _(Soll) in FIG. 3c in the case of a lower actual vehicle deceleration z_(Ist).

If as a result of the differential slip control or for other reasons the second region B2 is exited towards the first region B1, the differential slip control is advantageously terminated in a pulsed manner and the rear axle braking pressure p_(HA) is increased.

According to one embodiment, for the case in which there is an actual differential slip DS_(Ist) in the third region B3 that is greater than the target differential slip DS_(Soll), i.e. the rear axle HA is over braked, the outlet valves 12, 14 are opened so that a decrease in pressure on the rear wheel brakes 5, 6 takes place in order to reach or fall below the maximum deceleration characteristic z_(Kmax) as fast as possible and to reduce the over braking of the rear axle HA.

An electronic braking force distribution or a braking pressure distribution is caused by the differential slip control depending on the axle loads on the front axle VA and on the rear axle HA as follows:

If the vehicle 200 is not loaded, i.e. a small weight force is acting on the rear axle HA, the rear wheels 1, 2 tend to lock for the same brake actuation or for a specified target vehicle deceleration z_(Soll) more than for a fully loaded vehicle 200 with greater weight on the rear axle HA. The result of this is that the rear wheels 1, 2 are pressed harder onto the ground at the high weight in the loaded state. The load has at least no significant effect on the front axle VA owing to the large weight of the engine.

The actual differential slip DS_(Ist) thus varies depending on the load on the vehicle 200 for a defined target vehicle deceleration z_(Soll). The differential slip controller cannot directly measure the weight force acting on the rear axle HA, but it always controls the ABS valves 11, 12, 13, 14 on the rear wheel brakes 5, 6 at the point in time at which the load-dependent actual differential slip DS_(Ist) differs from the target differential slip DS_(Soll) by at least the tolerance DS_(T). Because the VD control device 15 does not control the front axle braking pressure p_(VA) during differential slip control, distribution of the braking forces to the vehicle axles VA, HA depending on the load is achieved indirectly by the differential slip control. Thus, the load on the vehicle 200 can be concluded from the actual differential slip DS_(Ist).

This can result in the situations shown in FIGS. 4a and 4b for different loads:

If the vehicle 200 is fully loaded, then the braking force is distributed approximately uniformly to the two vehicle axles HA, VA, as represented in the exemplary embodiment of FIG. 4a . I.e., depending on the target vehicle deceleration z_(Soll), the front and rear wheel brakes 5, 6, 7, 8 are subjected to about the same braking pressure p_(VA), p_(HA) to cause an actual vehicle deceleration z_(Ist), which for z_(Soll)=100% for example corresponds to z_(ist)=1 g (g=9.81 m/s²) if the front and rear wheel brakes 5, 6, 7, 8 are each subjected to a braking pressure of p_(VA)=p_(HA)=10 bar.

If the vehicle 200 is less than fully loaded, for example is not loaded, as represented in the exemplary embodiment in FIG. 4b , and has a mass in the unladen state that is approx. a third of the mass of the fully loaded vehicle 200, as shown in FIG. 4b a greater part of the braking is effected by the front wheel brakes 7, 8. In this example, for a target vehicle deceleration z_(Soll) of 100% (corresponds to 1 g with g=9.81 m/s² according to the above example), a front axle braking pressure p_(VA) of 6 bar and a rear axle braking pressure p_(HA) of only 1.2 bar are set in order to achieve an actual vehicle deceleration z_(Ist) of 100% corresponding to the target vehicle deceleration z_(Soll) as in FIG. 4a for the fully loaded state. The front axle VA is slowed less and the rear axle HA is slowed considerably less, whereby premature locking at low load of the front axle VA and in particular of the rear axle HA can be avoided.

The maximum deceleration characteristic curve z_(Kmax) and the minimum deceleration characteristic curve n(min can be stored in the VD control device 15, wherein at least the maximum deceleration characteristic curve z_(Kmax) can vary depending on the load. In this case, the maximum deceleration characteristic curve z_(Kmax) shifts as indicated in dashed form in FIG. 2 to higher actual vehicle decelerations z_(Ist), so that the third region B3 is reduced. I.e., for a certain target vehicle deceleration z_(Soll), the front axle braking pressure p_(VA) is first maintained at a higher actual vehicle deceleration z_(Ist). The rear axle braking pressure p_(HA), which can be matched to the front axle braking pressure p_(VA) by the differential slip control according to slip difference criteria, also increases as a result at a defined target vehicle deceleration z_(Soll). The vehicle 200 is thus decelerated more in the unladen state for the same target vehicle deceleration z_(Soll) than in the loaded state. Thus, the driver can be notified of the load state owing to the shift of the maximum deceleration characteristic curve z_(Kmax), so that the driver can react accordingly.

For measuring the load state, an ECAS system can be provided that controls the pneumatic suspension of the vehicle 200 electronically and for this purpose comprises a displacement sensor that is not shown that measures the displacement between the rear axle HA and a reference point on the vehicle 200 depending on the load. Furthermore, a mechanical-electronic axle load-dependent braking pressure distributor (ALB) can also produce the value of the load that is also controlled by a displacement sensor or a pressure sensor. Both systems, i.e. ECAS and ALB, can for example transmit the load state detected by said systems via the CAN bus to the VD control device 15, which selects the maximum deceleration characteristic curve z_(Kmax) depending thereon.

It is however also possible that the VD control device 15 determines the load state itself. For this purpose, the load can be determined using the measured actual differential slip DS_(Ist) as described above and the load state can be determined therefrom. Alternatively, a further displacement sensor can also be disposed in the vehicle 200 between a frame that is fixed relative to the vehicle and the vehicle axles HA, VA, the load-dependent displacement of which can be detected by the VD control device. The load on the vehicle can be concluded from the displacement.

Alternatively, engine performance data and vehicle acceleration data can be transmitted by an engine control unit that is not shown via the CAN bus, from which the VD control device 15 can conclude a mass of the vehicle 200 and hence an additional load compared to an empty weight of the vehicle 200. During this, it can also be taken into account whether a trailer is coupled to the vehicle 200, which can be proportionately allotted to the determined mass of the vehicle 200 and which can additionally influence the driving behavior at the rear axle HA. Said information can be transmitted via the CAN bus by a trailer control device. Furthermore, the behavior of a drive slip controller can be taken into account, from which a load state can also be determined.

A method according to an embodiment of the invention for electronic braking pressure control can be carried out according to FIG. 5 as follows. In a step St0, the braking is started, for example by actuation of the brake valve 9.2 by the driver. In a first step St1, the target vehicle deceleration z_(Soll) is detected with the braking value transducer 9.3 depending on the actuation of the brake valve 9.2, wherein either an actuation travel of the brake valve 9.2 or a rear axle or front axle braking pressure p_(HA), p_(VA) controlled by actuation of the brake valve 9.2 is detected.

In a second step St2, at the start of braking, i.e. when there are still no braking forces acting, an offset differential slip DS_(Off) is determined. Said offset differential slip DS_(Off) can differ from zero owing to an engine drag torque or wear on the wheels 1, 2, 3, 4 and hence varying wheel circumferences.

In a third step St3, depending on the load on the vehicle 200, a stored maximum deceleration characteristic curve z_(Kmax) is selected. Furthermore, a minimum deceleration characteristic curve z_(Kmin) is also determined. For the detected target vehicle deceleration z_(Soll), values for the maximum deceleration z_(max) and the minimum deceleration z_(min), on which the subsequent electronic vehicle deceleration control is based, thus result from the characteristic curves z_(Kmax), z_(Kmin).

In a fourth step St4, the actual vehicle deceleration z_(Ist) is detected. This results for example from an acceleration sensor that is not shown or from the derivative of a determined vehicle reference speed v_(ref), which can be determined for example from an average value of the revolution rates of the wheels 1, 2, 3, 4. Furthermore, in a step St4.1 it is determined whether a braking process or a slowing down process is occurring, i.e. whether the actual vehicle deceleration z_(Ist) is increasing or decreasing and whether the driver has demanded this by increasing or reducing the target vehicle deceleration z_(Soll).

In a fifth step St5, the actual vehicle deceleration z_(Ist) defined in the fourth step St4 by the VD control device 15 as a function of the target vehicle deceleration z_(Soll) is assigned to a region B1, B2, B3 of the graph according to FIG. 2, or a comparison is made as to whether the actual vehicle deceleration z_(Ist) is greater than or less than the maximum deceleration z_(max) or the minimum deceleration z_(min).

If the actual vehicle deceleration z_(Ist) lies in the first region B1, i.e. if it is less than the minimum deceleration z_(min), in a subsequent step St6.1 the inlet valves 11, 13, 21, 23 of the rear and front wheel brakes 5, 6, 7, 8 are opened and the outlet valves 12, 14, 22, 24 are closed, so that both the rear axle braking pressure p_(HA) and also the front axle braking pressure p_(VA) are increased.

If the actual vehicle deceleration z_(Ist) lies in the second region B2, i.e. if it is less than the maximum deceleration z_(max) and greater than the minimum deceleration z_(min), in a step St6.2 the inlet valves 11, 13 of the rear wheel brakes 5, 6 are actuated depending on the actual differential slip DS_(Ist) that is preferably determined during the entire braking process by the VD control device 15, such that the actual differential slip DS_(Ist) corresponds to the target differential slip DS_(Soll) that is stored in the VD control device 15 to within the tolerance DS_(T), which at least at the start of braking is the same as the offset differential slip DS_(Off) determined in the second step St2 and which can vary during braking as a function of the acceleration of the vehicle. In this case, the rear axle braking pressure p_(HA) can either be maintained or increased. According to one embodiment, a pressure reduction can also be provided in the second region B2 by the differential slip control by actuating the outlet valves 12, 14 of the rear wheel brakes 5, 6.

The inlet valves 21, 23 and the outlet valves 22, 24 of the front wheel brakes 7, 8 are controlled in the second and in the third region B2, B3 preferably depending on whether a braking process or a slowing down process was detected in the step St4.1, wherein the front axle braking pressure p_(VA) in the case of a braking process in the second region B2 is maintained or increased in a step St6.3, and is maintained in the third region B3 according to a step St6.5, and is limited in the case of a slowing down process in the second and in the third region B2, B3, i.e. maintained or reduced (steps St6.4 and St6.6), depending on whether the target vehicle deceleration z_(Soll) has already been reached or not.

The inlet valves 11, 13 and the outlet valves 12, 14 of the rear wheel brakes 5, 6 are also controlled in the third region B3 depending on whether a braking process or a slowing down process is occurring, wherein the rear axle braking pressure p_(HA) is maintained during a braking process according to step St6.5 and during a slowing down process according to step St6.7 is limited, i.e. maintained or reduced, depending on whether the target differential slip DS_(Soll) has already been reached or not, wherein the rear axle braking pressure p_(HA) is reduced in the case of an actual differential slip DS_(Ist) that is greater than the target differential slip DS_(Soll), i.e. in the case of relative over braking.

Following the steps St6.1, St6.2, St6.3, St6.4, St6.5, St6.6 or St6.7, the electronic vehicle deceleration control starts again within a braking process with the fourth step St4. Thus, in the case of continuous braking, the detected actual vehicle deceleration z_(Ist) is associated with a region B1, B2, B3 and the rear axle braking pressure p_(HA) and the front axle braking pressure p_(VA) are adjusted accordingly.

The differential slip control in the step St6.2 proceeds according to FIG. 6 as follows: In a step St100, the actual differential slip DS_(Ist) is compared with the target differential slip DS_(Soll), which is stored in the VD control device 15 as represented in FIG. 3 and with which the offset differential slip DS_(Off) determined in step St2 of the vehicle deceleration control is taken into account.

If the actual differential slip DS_(Ist) is greater than the target differential slip DS_(Soll), the rear axle HA is over braked relative to the front axle VA. If the actual differential slip DS_(Ist) is above the tolerance DS_(T) in this case, the VD control device 15 causes limiting of the rear axle braking pressure p_(HA) in a step St101.1, so that the braking force of the wheel brakes 5, 6 on the rear axle HA is not increased further. If the actual differential slip DS_(Ist) is less than the target differential slip DS_(Soll) or less than the tolerance DS_(T), an increase of the rear axle braking pressure p_(HA) is demanded by the VD control device 15 in a step St101.2, because the rear axle HA is under braked relative to the front axle VA. If the actual differential slip DS_(Ist) equals the target differential slip DS_(Soll) within the tolerance DS_(T), the method starts from the beginning; the tendency to locking of the rear axle HA has been adapted to the tendency to locking of the front axle VA.

FIGS. 7a and 7b show by way of example a maximum deceleration characteristic curve z_(Kmax) with an upper tolerance z^(u) _(T) and a lower tolerance z^(l) _(T) for a braking process (FIG. 7a ) and a slowing down process (FIG. 7b ). It is started at point P1, which corresponds to a braking situation, in which the driver specifies a target vehicle deceleration z_(Soll). At point P1 the maximum deceleration characteristic curve z_(Kmax) is reached, i.e. both the rear axle braking pressure p_(HA) and also the front axle braking pressure p_(VA) are limited by the vehicle deceleration control, preferably maintained, by shutting off the inlet valves 11, 13, 21, 23 and the outlet valves 12, 14, 22, 24 independently of the detected actual differential slip DS_(Ist), the actual vehicle deceleration z_(Ist) lies in the third region B3.

If the driver operates the brake pedal harder, the braking situation shifts from point P1 to point P2, wherein the actual vehicle deceleration z_(Ist) is below the lower tolerance z^(l) _(T); the actual vehicle deceleration z_(Ist) lies in the second region B2. Because this is a braking process, the front axle braking pressure p_(VA) in said second region B2 is either maintained or increased, depending on whether the target vehicle deceleration z_(Soll) has already been reached. The rear axle braking pressure p_(HA) is controlled as a function of the detected actual differential slip DS_(Ist). If the actual differential slip DS_(Ist) lies above the target differential slip DS_(Soll), the rear axle HA is over braked relative to the front axle VA, so that the rear axle braking pressure p_(HA) is maintained and the front axle braking pressure p_(VA) is increased. If the actual differential slip DS_(Ist) shifts to below the target differential slip DS_(Soll), the rear axle HA is under braked and an increase of the rear axle braking pressure p_(HA) is demanded. If on reaching the point P2 the actual differential slip DS_(Ist) is already less than the target differential slip DS_(Soll), both the rear axle braking pressure p_(HA) and also the front axle braking pressure p_(VA) are increased.

By said measures, the vehicle 200 is decelerated in such a way that the point P3 is reached, which lies on the maximum deceleration characteristic curve z_(Kmax), so that both the rear axle braking pressure p_(HA) and also the front axle braking pressure p_(VA) are again limited. In the case of a further increase of the actuation of the brake pedal, point P4 is reached, at which the method is repeated as at point P2.

If the driver releases the brake pedal slightly at the point P5, for example after a braking process, then the braking situation shifts according to FIG. 7b to point P6, which lies above the upper tolerance z^(u) _(T), i.e. an increase of the front axle braking pressure p_(VA) and the rear axle braking pressure p_(HA) is not enabled. If the detected actual differential slip DS_(Ist) at said point P6 is less than the target differential slip DS_(Soll), the rear axle braking pressure p_(HA) is initially maintained and the front axle braking pressure p_(VA) is reduced according to the target vehicle deceleration z_(Soll) specified by the driver, in order to compensate the under braking of the rear axle HA relative to the front axle VA. If the actual differential slip DS_(Ist) at the point P6 is greater than the target differential slip DS_(Soll), both the front axle braking pressure p_(VA) and also the rear axle braking pressure p_(HA) are reduced, in order to correspond to the driver's wishes, until the actual differential slip DS_(Ist) again corresponds to the target differential slip DS_(Soll) within the tolerance D_(T).

By said measures, the braking situation shifts from point P6 to point P7, at which to reach the point P8 the measures described above are repeated if the driver releases the brake pedal further, i.e. the target vehicle deceleration z_(Soll) itself is actively reduced.

In the case of a change from a braking process to a slowing down process, to reduce pressure a longer actuation time of the outlet valves 12, 14, 22, 24 can be selected once, and in the case of a change from a slowing down process to a braking process, to increase pressure a longer actuation time of the inlet valves 11, 13, 21, 23 can be selected once in order to overcome hysteresis behavior during the reversal of the braking actions of a brake.

LIST OF REFERENCE NUMERALS

1, 2 rear wheels

3, 4 front wheels

5, 6 rear wheel brakes

7, 8 front wheel brakes

9.1 brake cylinder

9.2 dual circuit brake valve

9.3 pressure sensor/braking value transducer

10 ABS control device

11, 13 ABS inlet valves of rear wheel brakes 5, 6

12, 14 ABS outlet valves of rear wheel brakes 5, 6

15 VD control device

21, 23 ABS inlet valves of front wheel brakes 7, 8

22, 24 ABS outlet valves of front wheel brakes 7, 8

100 brake system

101 EBS control device

200 vehicle

B1, B2, B3 first, second and third regions

DSR differential slip control

DS_(Ist) actual differential slip

DS_(Soll) target brake slip

DS_(Off) offset differential slip

DS_(HA) rear axle brake slip

DS_(VA) front axle brake slip

DS_(T) tolerance value of differential slip

HA rear axle

p brake pressure

p_(VA) front axle brake pressure

p_(HA) rear axle brake pressure

VA front axle

v_(VA) rotation rate of front axle VA

v_(HA) rotation rate of rear axle HA

z_(Kmax) maximum deceleration characteristic curve

z_(Kmin) minimum deceleration characteristic curve

z_(max) maximum deceleration

z_(min) minimum deceleration

z_(Soll) target vehicle deceleration

z_(Ist) actual vehicle deceleration

z^(u) _(T) upper tolerance (vehicle deceleration)

z^(l) _(T) lower tolerance (vehicle deceleration) 

1. A method for controlling a vehicle deceleration depending on a differential slip between two vehicle axles in a vehicle with an ABS brake system, the method comprising: detecting at least one of a target vehicle deceleration specified by the driver and an actual vehicle; and controlling a braking pressure on wheel brakes of a vehicle axle to be controlled by the actuation of ABS brake valves in such a way that the braking pressure on the wheel brakes of the vehicle axle to be controlled is controlled depending on a detected actual differential slip, so that the actual differential slip corresponds to a target differential slip, wherein the actual differential slip indicates a difference in a rotational behavior of the vehicle axle to be controlled relative to a further vehicle axle, and wherein the target differential slip is dependent on at least one of the detected actual vehicle deceleration and the detected target vehicle deceleration, wherein the braking pressure on the wheel brakes of the vehicle axle to be controlled is limited if the vehicle axle to be controlled is over braked relative to the further vehicle axle, and wherein the target differential slip is reduced when increasing the actual vehicle deceleration and/or increasing the target vehicle deceleration towards synchronous wheel rotational behavior of the wheels of the vehicle axle to be controlled and the wheels of the further vehicle axle, so that the braking pressure on the vehicle axle to be controlled is already limited for a lower relative over braking of the vehicle axle to be controlled.
 2. The method as claimed in claim 1, wherein the braking pressure on the wheel brakes of the vehicle axle to be controlled is increased if the vehicle axle to be controlled is under braked relative to the further vehicle axle.
 3. The method as claimed in claim 1, wherein the actual differential slip is calculated from a difference of a determined rear axle brake slip minus a determined front axle brake slip normalized to the rear axle brake slip, or a difference of a determined axle speed of the front axle minus a determined axle speed of the rear axle normalized to the axle speed of the front axle, wherein the rear axle is the vehicle axle that is driven by an engine.
 4. The method as claimed in claim 1, wherein if the vehicle axle to be controlled is the rear axle, the braking pressure on rear wheel brakes is limited if the actual differential slip is greater than the target differential slip, and is increased if the actual differential slip is less than the target differential slip, and wherein if the vehicle axle to be controlled is the front axle, the braking pressure on the front wheel brakes is limited if the actual differential slip is less than the target differential slip and is increased if the actual differential slip is greater than the target differential slip.
 5. The method as claimed in claim 1, wherein the braking pressure on the wheel brakes of the vehicle axle to be controlled is controlled depending on the detected actual differential slip, so that the actual differential slip corresponds to a target differential slip within a tolerance, wherein the tolerance lies below and/or above the target differential slip.
 6. The method as claimed in claim 1, wherein a maximum deceleration and a minimum deceleration are each specified depending on the detected target vehicle deceleration, and wherein the braking pressure on wheel brakes of a first and a second vehicle axle are controlled depending on the detected actual vehicle deceleration by the actuation of ABS brake valves, wherein controlling the braking pressure comprises: increasing the braking pressure on the wheel brakes of the first and second vehicle axles if the actual vehicle deceleration is less than the minimum deceleration in order to achieve a minimum braking effect, limiting the braking pressure on the wheel brakes of the first and the second vehicle axles if the actual vehicle deceleration is greater than the maximum deceleration to limit the braking effect of the first vehicle axle, and controlling, depending on the detected actual differential slip, the braking pressure on the wheel brakes of the second vehicle axle is if the actual vehicle deceleration is less than the maximum deceleration and greater than the minimum deceleration so that the actual differential slip corresponds to the target differential slip, wherein the first vehicle axle is the further vehicle axle and the second vehicle axle is the vehicle axle to be controlled.
 7. The method as claimed in claim 6, wherein the maximum deceleration is specified using a maximum deceleration characteristic curve which represents a dependency of the maximum deceleration on the target vehicle deceleration and the minimum deceleration is specified using a minimum deceleration characteristic curve which represents a dependency of the minimum deceleration on the target vehicle deceleration.
 8. The method as claimed in claim 7, wherein the maximum deceleration characteristic curve is dependent on a load on the vehicle, wherein the maximum deceleration characteristic curve rises more steeply for a smaller load, so that the braking pressure is limited for a smaller load on the vehicle for higher detected actual vehicle decelerations in contrast to a large load on the vehicle.
 9. The method as claimed in claim 8, wherein the load on the vehicle is measured with an axle load sensor or is derived from the detected actual differential slip.
 10. The method as claimed in claim 6, wherein the maximum deceleration characteristic curve has a harmonic profile without steps for limiting the braking pressure without decelerating the vehicle in a jerky manner.
 11. The method as claimed in claim 6, wherein the braking pressure on the wheel brakes of the first vehicle axle is maintained or increased during a braking process, and is maintained or reduced during a slowing down process, if the actual vehicle deceleration is less than the maximum deceleration and greater than the minimum deceleration.
 12. The method as claimed in claim 6, wherein the braking pressure on the wheel brakes of the second vehicle axle is reduced depending on the actual differential slip if the actual vehicle deceleration is greater than the maximum deceleration and the second vehicle axle is over braked relative to the first vehicle axle.
 13. The method as claimed in claim 1, wherein the vehicle axle to be controlled is a rear axle of the vehicle or a front axle of the vehicle and the further vehicle axle is a front axle of the vehicle if the vehicle axle to be controlled is a rear axle of the vehicle or a rear axle of the vehicle if the vehicle axle to be controlled is a front axle of the vehicle.
 14. The method as claimed in claim 1, wherein in the case of limiting of the braking pressure on the wheel brakes of the vehicle axle to be controlled or the further vehicle axle, the braking pressure is only maintained or reduced and is not increased.
 15. The method as claimed in claim 1, wherein for the case in which the target differential slip is a function of the actual vehicle deceleration and the target vehicle deceleration, the target differential slip decreases for a smaller actual vehicle deceleration if the actual vehicle deceleration is greater than the target vehicle deceleration, and decreases for a higher actual vehicle deceleration if the actual vehicle deceleration is less than the target vehicle deceleration relative to the case in which the actual vehicle deceleration corresponds to the target vehicle deceleration within a tolerance.
 16. A device for electronically controlling a vehicle deceleration in a vehicle with an ABS brake system, the device being configured to: detect at least one of a target vehicle deceleration specified by the driver and an actual vehicle deceleration, and actuate ABS brake valves on wheel brakes of a vehicle axle to be controlled depending on a detected actual differential slip in such a way that the actual differential slip corresponds to a target differential slip, wherein the actual differential slip indicates a difference in a rotational behavior of the vehicle axle to be controlled from a further vehicle axle, and wherein the target differential slip is dependent on the detected actual vehicle deceleration and/or the detected target vehicle deceleration.
 17. The device as claimed in claim 16, wherein the second vehicle axle is a rear axle and the first vehicle axle is a front axle of the vehicle.
 18. A vehicle with a brake system, the brake system comprising: a brake valve for controlling a braking pressure on wheel brakes of the vehicle, a braking value transducer for detecting a target vehicle deceleration, a sensor for detecting an actual vehicle deceleration, ABS brake valves, and a device for electronically controlling a vehicle deceleration in a vehicle with an ABS brake system, the device being configured to: detect at least one of a target vehicle deceleration specified by the driver and an actual vehicle deceleration, and actuate ABS brake valves on wheel brakes of a vehicle axle to be controlled depending on a detected actual differential slip in such a way that the actual differential slip corresponds to a target differential slip, wherein the actual differential slip indicates a difference in a rotational behavior of the vehicle axle to be controlled from a further vehicle axle, and wherein the target differential slip is dependent on the detected actual vehicle deceleration and/or the detected target vehicle deceleration. 